1. Field of the Invention
This invention relates to nitrogen fertilizer solutions produced from urea and formaldehyde. More particularly it relates to a method for producing storage stable, controlled release, nitrogen solution fertilizers by a two stage process, and the product derived from that method.
2. Description of the Prior Art
Urea formaldehyde condensation polymers have been used for many years to provide fertilizers which release nitrogen at rates to adequately feed growing plants without damaging either the foliage or root systems of the plants. Urea formaldehyde solid fertilizers have been widely used but are difficult to apply evenly and release their nitrogen contents in an incomplete and sometimes unpredictable manner.
It is well known that the reaction of urea with a molar excess of formaldehyde for moderate periods of time in the presence of bases produces methylolureas at ambient or moderate temperatures. This reaction is the basis for the production of urea formaldehyde concentrates, such as "UFC-85" widely marketed today and patented by H. M. Kvalnes in U.S. Pat. Nos. 2,467,212 (1949); reissue U.S. Pat. No. Re. 23,174 (1949).
The art contains numerous efforts to produce urea formaldehyde based liquid fertilizer products which are easy to store and apply evenly. The recent prior art contains several disclosures of urea formaldehyde based clear aqueous solutions, and aqueous liquid dispersions of insoluble urea formaldehyde polymers.
G. H. Justice et al in U.S. Pat. No. 3,462,256 disclosed a process for producing urea formaldehyde concentrates which are storage stable for 20-25 days at 25.degree. C. and 7 days at 0.degree. C. Justice prepares urea formaldehyde solutions by reacting an aqueous solution of urea (U) and formaldehyde (F) in a 1 to 2:1 U:F mol ratio with 0.3 to 6.0 percent ammonia at a strong alkali-induced pH of 8.5 to 10.0 and 75.degree. to 100.degree. C. until at least 90 percent of the formaldehyde is reacted with at least 60 percent of it in the form of methylolurea. Alkali addition is then terminated and heating is continued until at least 50 percent of the formaldehyde is converted to methylene urea, as determined by the difference between total formaldehyde and free formaldehyde plus methylolurea. Unfortunately, the product of the Justice process contains as much as 40 percent of the formaldehyde moeities as methylolurea, and this continues to react with the methylene urea to form water insoluble precipitates so that the storage life must be limited to a month or less at ambient temperatures and a week under freezing temperature.
E. F. Hawkins in U.S. Pat. No. 4,544,005 discloses a controlled release nitrogen solution described as a triazone solution containing about 30 parts triazone, 40 parts urea, up to 3 parts methylene diurea, up to 6 parts monomethylolurea and hexamethylenetetramine, wherein the triazones amount to an 80/20 mixture of S-tetrahydrotriazone and N-hydroxymethyl formamide triazone. Hawkins provides a two stage process for heating aqueous urea and formaldehyde in a 1 to 2:1 mol ratio at 85.degree. to 90.degree. C. at a strong caustic induced pH of 8.5 to 9.2 in the presence of 0.20 to 0.38 mols of ammonia per mol of formaldehyde for a period of 25 to 60 minutes and then maintaining the temperature and terminating the addition of strong caustic for a period of 30 to 60 minutes. The relatively high urea concentration required in the Hawkins product increases the phytotoxicity of any fertilizer solutions produced, and decreases the value of that portion of the nitrogen that is protected.
W. P. Moore in U.S. Pat. No. 4,244,727 reissue U.S. Pat. No. Re. 31,801 teaches a process for reacting aqueous urea and formaldehyde at a U:F mol ratio of 1.5 to 2.0:1 at 75.degree. to 90.degree. C. while maintaining pH between 8.5 and 9.8 in the presence of alkali to produce a storage stable fertilizer solution comprising methylolurea. Aqueous methylolurea exhibits lower phytotoxicity toward plants than free urea, and is storage stable in base solutions for long periods of time but releases its nitrogen more rapidly than methylene ureas.
In U.S. Pat. No. 4,304,588, W. P. Moore discloses a process for preparing storage stable aqueous concentrates of nitrogenous foliar feeds comprising monomethylolurea with monomethylene diurea concentration limited to 5 percent. The process consists of reacting aqueous urea and formaldehyde in the presence of an alkali metal carbonate and hexamethylene tetramine at 85.degree. to 95.degree. C. with a U:F mol ratio between 1.4 and 1.9, with at least 50 percent of the formaldehyde reacted to form methylolurea. A pH of 8.5 to 10.5 is prescribed for the reaction and the product.
T. T. Grace in U.S. Pat. No. 4,409,015 discloses a two stage process for preparing water insoluble ureaform solids dispersed in aqueous urea and soluble ureaform compounds. Grace teaches preparation of a preresin by heating a high formaldehyde to urea molar mixture in the presence of acid until a desired level of polymerization is reached, as indicated by methanol insolubility and viscosity measurements. More urea is then added, and acid and further polymerization is then carried out at lower temperatures until the desired amount of water insoluble polymers are formed. Although the desirable slower release of methylene ureas is obtained, settling of the water insoluble solids during storage and clogging of spray nozzles by the solids during applications can create serious problems in field operations.
Although much progress in the area of urea formaldehyde liquid fertilizers has been made in the prior art, no disclosure has been made of a simple and easily controlled, two stage method for the preparation of a storage stable clear controlled release nitrogen fertilizer solution comprising polymethylene ureas.
It is a primary object of this invention to provide a readily controlled method for preparing a storage stable, controlled release, polymethylene urea nitrogen fertilizer solution from urea and formaldehyde.
It is another object of this invention to provide a storage stable controlled release polymethylene urea nitrogen fertilizer solution containing less than 40 percent of its nitrogen in the form of urea, and less than 6 percent of its nitrogen as methylene diurea.